DE2638270A1 - PROCESS FOR MANUFACTURING LARGE AREA, CANTILEVER PLATES MADE OF SILICON - Google Patents

Description

Process for the production of large, self-supporting plates made of silicon

The invention relates to a process for the production of large, Self-supporting plates made of silicon, in which silicon is deposited from the gas phase on a suitable substrate body.

Solar cells, as they have been used to generate electricity in space travel, are far too expensive for a wide range of terrestrial applications. A manufacture of solar cells from high-purity, single-crystal silicon rods, in which approx. 70 % of the material is lost in the labor and material cost-intensive processing of these rods, can therefore be ruled out from the outset for mass production.

For this reason, work has been going on for a number of years to make it cheaper to use available, largely polycrystalline silicon as the base material, albeit with the abandonment of monocrystalline nature the efficiency of solar cells made from this type of material is also obviously falling sharply and so far only at a few percent.

A first breakthrough to cheap, inexpensive silicon base material on the other hand is included in the procedure according to DT-OS 25 08 803 which silicon is cast directly into thin plates under the action of a temperature gradient. For this material with larger,

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In monocrystalline areas, solar cells with efficiencies over 5 % can already be produced.

According to research by T.L. Chu et al manage solar cells produced by depositing silicon from silane or trichlorosilane on graphite disks in an epireactor. The setting High substrate temperatures, low deposition rates and the composition of the deposition gas essentially determine the efficiency of the solar cells made from it, which is determined by a heat treatment of the deposited silicon layer can still be increased, but even then is only at a maximum of 5% (Journal of Electrochem. Society, 1976, pages 10-110).

The present invention is based on the object of a method to find for the production of solar cell material, in which the labor-intensive and cost-driving process steps, according to known processes between the production of the semiconductor base material and the usable solar cell can be reduced.

This object is achieved in that silicon is deposited on a graphite substrate body heated to deposition temperature from a gaseous compound to which a dopant is additionally admixed up to a thickness of 200 to 650 μm and subsequently 40 to 100% of this siliconura layer from the free The surface is melted, caused to solidify again by setting a suitable temperature gradient from the substrate body and is then separated again from the substrate body.

Graphite foils, for example, are suitable as graphite substrate bodies, which, according to the manufacturer, are made of pure, well-ordered graphite, with chemical and thermal treatment the distances between the layers in the crystal lattice of graphite are widened to a multiple of the normal value of 35 Å. The resulting bulk weight of worm-shaped individual parts is then compacted to the end product on calenders or presses,

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the layers of the graphite lattice solely through the use of mechanical Pressure are firmly connected to each other again.

However, graphite plates are also very suitable, in particular those which, due to their pretreatment, are anisotropic with a view to aligning the graphite planes as parallel as possible to the two opposite largest boundary surfaces of the are plate-shaped bodies. Only this two-dimensional preferential arrangement is important, since the orientation is perpendicular to the Six-membered planes of the hexagonal graphite lattice is irrelevant. Graphite plates or other substrates are therefore particularly preferred made of ceramic, for example, on which a pyrographite layer is applied, since such pyrographite layers are equal to the desired Preferred orientation from mixtures of hydrocarbons with favorable about 1 to 10 carbon atoms and an inert gas or For example, hydrogen from a methane-hydrogen mixture let pyrolytically separate.

This preferred orientation means that it is easy to split of the graphite between the individual six-ring planes facilitates the later lifting of the silicon plates deposited on it.

Application is recommended before the actual silicon deposition a thin, preferably 1 to 5 .mu.m thick intermediate layer of insulating character. The intermediate layer has the advantage that the graphite substrate body does not contact silicon directly during the deposition comes into contact and the deposited silicon layer can later be easily separated again. Next to that comes the intermediate layer a getter effect, so that quickly diffusing impurities in the intermediate layer during the cooling process accumulate.

The material for the intermediate layer is generally oxidic, nitridic, carbidic, finely divided to amorphous substances, for example amorphous silicon, silicon oxide, silicon carbide or, preferably, silicon nitride. ...

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The intermediate layer can be simply painted on, for example or spraying corresponding substances onto the substrate body or also by pyrolytic decomposition of, for example Prepare silanes or silicic acid esters in the presence of oxygen, ammonia or suitable carbon compounds.

An intermediate layer of finely divided, amorphous silicon can also be used are formed virtually in situ during the deposition of the silicon layer directly on the untreated substrate body in the manner that a few microns of the layer adjoining the substrate are excluded from subsequent process steps, i.e. not melted and be recrystallized.

The preferred application of a silicon mitride interlayer can for example, by applying finely divided silicon to the substrate body and subsequent nitriding. After a In a preferred embodiment, however, the silicon nitride layer is made from the gas phase, from a mixture of a silane or halosilanes with ammonia and hydrogen in excess at temperatures from 750 to 1100.degree. C., preferably 950 to 1050.degree manufactured. Suitable gas mixtures here would be monosilane / ammonia / hydrogen or tetramethylsilane / ammonia / Hydrogen, with the latter usually forming a mixed phase of Siliciuracarbidnitrid, the same as an intermediate layer is usable.

For the deposition of the silicon layer, the substrate body, with or without an intermediate layer, depending on the * type of deposition gas, to the required deposition temperature of 800 to 1400 ° C. and with the preferred use of trichlorosilane / hydrogen heated to approximately 1000 to 1250 ° C.

In general, for example, silicon hydrogen, Monochlorosilane, dichlorosilane, trichlorosilane, tetrachlorosilane, hexachlorodisilane alone or as a mixture, optionally with an addition of hydrogen or inert gases such as argon or helium can be used.

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Dopants are added to the deposition gas during deposition such as phosphorus, arsenic or antimony, which require an n-line or preferably suitable boron, aluminum, Gallium or indium compounds that lead to a p-type line.

The deposition is broken off after reaching the desired layer thickness of about 200 to 65Oyum, preferably 300 to 500um, and 40 to 100 % with respect to the layer thickness of the silicon layer formed is melted from the surface.

The silicon layer is expediently melted by supplying energy from the substrate body and at the same time by radiant heat more suitable, installed over the surface of the silicon layer Heating lamps.

The basic load of the required energy is preferably introduced by appropriate heating of the substrate body, so that the energy required to melt the silicon layer from the surface, which is generated by the corresponding energy above the silicon layer attached radiation sources is generated, rainiraalisiert will. This measure allows the use of cheaper heating lamps that are commercially available at any time. In this preferred embodiment, the graphite substrate body is, for example, by direct The passage of current is heated to such an extent that the adjacent silicon layer is brought to a temperature of, for example, 1100 to 1250 ° C will. The residual heat, which is required to melt the silicon layer at 1410 ° C, can then be generated by conventional heating lamps, whose radiation acts on the surface of the silicon layer can easily be applied.

The graphite substrate body can of course also be heated by contact Heat over the support, and the silicon layer can also be heated by means of a suitable induction heating coil possible, whereby the heating of the graphic substrate body is not absolutely necessary ^ 'i *

In the event that there is no intermediate insulating layer before the silicon dioxide deposition was applied to the substrate body, there is also the possibility of the silicon layer with a glow discharge under

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Normal pressure in a hydrogen or at least strongly hydrogenated To melt the inert gas atmosphere in such a way that the electrically conductive graphite substrate body is connected as a cathode and a metal bracket acting as an anode is guided over the silicon surface at a suitable distance. This variant of the However, the method can only be used when the entire silicon layer is to be melted down to the substrate body. on a separate substrate heater can of course be dispensed with.

The liquid silicon layer or the melted part of the The silicon layer is subsequently formed by setting a vertical direction from the substrate body to the surface of the silicon layer Temperature gradients and the temperature level described thereby below the melting point of the silicon for directional resolidification brought. This is done, for example, in such a way that only the substrate heating is slowly turned down, as a result of which solidification of the silicon layer from the substrate body to the free surface is effected if the lamps alone are not sufficient, to keep the silicon layer molten. The solidification process can of course be shortened by also the radiation energy of the heating lamps is regulated down, but in such a way that it is in the liquid silicon layer an upward vertical temperature gradient of the order of about 30 to 100 ° C is established. An accelerated one The substrate body can also be cooled by passing a cooling gas, for example argon, past it.

According to the preferred embodiment of the method, a pn junction is formed in the surface of the silicon dioxide layer in a subsequent method step by removing the melted Silicon layer on the surface, ijfi-presence ", of a dopant, which silicon is doped in the opposite direction to that of the dopants in the adjoining Siliciuraschicht is melted again. If, for example, the silicon layer was p-doped in accordance with the preferred embodiment, the remelted layer would be used Surface, for example, as much phosphorus, arsenic or antimony as is necessary to achieve an n-conduction of this layer is to bring in.

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Another possibility is to use the silicon layer only except for a thin liquid surface film to solidify again to bring and before solidification also this thin surface layer to diffuse a corresponding dopant.

Instead of the surface of the silicon layer ura, for example 0.3 to 1.5 to melt in order to get the corresponding from the gas phase To introduce dopant, so for example in the case of phosphorus to pass over phosphine, it is also possible to use the solid silicon surface to be coated with an appropriate dopant and this then diffuse superficially into the silicon layer under the action of heat. Another possibility is also the dopants by ion implantation in the surface area to build a solid silicon layer.

After cooling to room temperature, the substrate body is removed from the The grown silicon layer is separated again. The separation can be done by mechanical means, for example with a thin cutting tool lifted the silicon layer from the substrate body. The bond between the silicon layer and the The substrate body is anyway due to the cooling process and the special structure of the graphitic substrate surface, especially at Installation of an insulating intermediate layer loosened to such an extent that separation does not cause any difficulties. Before the graphite substrate body is reused, it is expedient to use a suitable brush or by sandblasting to remove any adhering residues the intermediate layer freed. In addition to dry cleaning, such residues can optionally also be cleaned with a suitable solvent removed.

It is favorable to have the substrate body made of graphite foil, for example designed as an endless belt, whereby the substrate surface that becomes free after the separation of the deposited silicon layer is repeated again and again is coated. This can also be done with graphite plates Carry out a continuous process advantageously in such a way that the individual plates coat-recrystallize the deposition system in a rhythm - Peeling - cleaning - coating constantly going through.

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If the process is not carried out continuously, but takes place the deposition, for example, on a very thin graphite foil, this can be done, for example, by simple sandblasting remove again.

In a preferred embodiment, the process is continuous carried out, as exemplified using the schematic representation is described.

By a suitable drive, for example a set of rollers are on a steel plate extending through the entire system 2 individual, butting, preferably pyrographite-coated graphite plates 3 continuously at one speed from 1 to 15 cm per minute, preferably 3 to 7 cm per minute funded by the plant. The steel plate 2 or a correspondingly dimensioned ceramic or quartz plate, which is supported by a Resistance heating installed underneath, for example in the form of heating coils or tubes, can be heated differently in sections is, corresponds in width to the width of the graphite plates 5 and is provided at the edges with an abutting edge, so that the graphite plates 3 with the formation of a continuous band-shaped separation surface be pressed over the steel plate 2 through the separator. The width of the plates 3 itself depends on the dimensioning of the system and corresponds essentially to the width of the separating caramers minus the wall thickness. Advantageous values for the width are approximately 5 to 20 cm, for the length likewise approximately 5 to 20 cm and for the height about 0.1 to 1 cm for a parallelogram in the direction of movement, preferably rectangular cross-section.

In addition to this special embodiment, the graphite plates can also be placed on a conveyor belt that can be heated differently in sections and conveyed through the system. Another Possibility is to have the graphite plates butt against each other to let the free edges slide through the system on two guide rails, in which case the heating is provided by direct current passage or contact-free by means of heating elements made of, for example, ceramic or fused silica, which are arranged below the graphite plates he follows.

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The graphite substrate plates 3 pass before entering the first separation chamber 4 a gas lock 5 provided with a pressure control valve? in which the surface of the substrate plates. 3 of an inert gas stream, for example argon, at a gas pressure of about 0.1 to 0.3 bar above the gas pressure in the subsequent first Separation chamber 4 is, is flowed around, whereby it is avoided that the separation gas from the separation chamber 4 into the outer space or Air from the outside space penetrates into the separation chamber 4.

In the following first separation chamber 4, by means of a '. {Slot-shaped double pipe, consisting of the inner Tube 6 and the outer jacket 7, a gas mixture of, for example, monosilane in favorably about 300 to 400 parts by volume of hydrogen through the inner tube 6 and ammonia undiluted or blown in up to 400 parts by volume of hydrogen through the outer jacket 7, where the molar ratio, based on the components monosilicon to ammonia, is expediently about 1 in 2 to 1 in 4.

From the slit-shaped double tube, the width of which corresponds essentially to the clear width of the separation chamber 4 and which is moderately with its gas outlet opening up to about 0.5 to 3 cm, preferably about 1 to 2 cm to the surface of the graphite substrate body reaches, the gas mixture is expediently blown in under a pressure of 1 to 2 bar, preferably 1.1 to 1.4 bar, the Gas flow is chosen so that during passage through the separation chamber 4 on the means of the heating device 8 on a temperature of about 750 to 1200 C, preferably 950 to 1000 C, heated strip of graphite plates 3 an intermediate layer of silicon nitride 9 in a thickness of 1 to 5 Λίφ »preferably 1.5 to 3 Λ" Β separates. The reaction gases exit the separation chamber 4 again through the gas discharge nozzle 10.

After this first separation chamber 4, which of course only then required if the application of an intermediate layer between the substrate body and the silicon layer to be deposited is intended, happened the strip of graphite plates 3, before entering the second separation chamber 11, an intermediate gas lock 12 provided with a pressure holding valve, in which an inert gas, such as in particular Argon, under a pressure which is expediently 0.1 to 0.3 bar above

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the pressure of the two adjoining and expediently at least approximately Deposition chambers 4 and 11 operated under the same gas pressure lies on the surface of the intermediate-coated strip of graphite plates 3 is blown to prevent the reaction and deposition gases from escaping to prevent from the two separation chambers.

The large separation chamber 11 is charged with the separation gas via the gas inlet connection 13. The separation gas is for example made up of about 80 to 95 parts by volume of hydrogen and 5 to 20 parts by volume of, for example, trichlorosilane, whereby the volume parts; should add up to 100, and hydrogen with a content of about 50 to 100 ppm diborane is added. In total, enough dopant is added until a gas is present in the deposition gas. The atomic ratio of boron to silicon is set from 1: 10 to 1: 10. The dimensioning of the gas inlet nozzle 13 is quite uncritical, provided that its cross-sectional area only allows the inlet of the separation gas under a pressure of about 1 to 2 bar, preferably 1.1 to 1.4 bar in such an amount that during the passage of the, by means of the heating device 14 to a temperature of about 1000 to 1250 ° C, preferably 1050 to 1150 ° C and optionally intercoated strip of graphite plates 3 doped through the large deposition chamber 11 in a thickness of 200 to 65O yura, preferably 300 to 500 is deposited on. The reaction gases and unconverted separation gas leave the large separation chamber 11 through the gas discharge nozzle 15.

After the coated strip of graphite plates 3 emerges from the large separation chamber 11, it enters the gas nozzle 16 with inert gas, for example argon, via the pressure holding valve provided with an overpressure of 0.1 to 0.3 bar compared to the pressure in the separation chamber 11 charged recrystallization chamber 17 Here the deposited silicon layer 18 is coated with a halogen rod lamp 191 extending over the entire bandwidth, its upwardly directed radiation via a reflector 20 with an ellipsoidal cross-section on a preferably approximately 1 to 2 mm narrow area 21 across the entire width of the silicon Focused graphite plates, as a result of which up to 100 % of this part of the silicon layer 18 is melted with respect to the layer height 4θ. The basic load of the energy supply is introduced via the substrate body, in such a way that this part of the belt is caused by the heating device 22

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is heated graphite plates 3 on a Teraparatur of advantageously approximately 1150 to 1250 ⁰ C.

An inert gas, for example at room temperature, is supplied through the gas nozzle 23, which is preferably attached below the strip of graphite plates 3 and thus argon at a temperature of about 25 C, blown as a cooling gas against the strip of graphite plates 3, whereby the heat of crystallization is dissipated and a directional growth of the re-solidifying Silicon layer is effected from the substrate body to the free surface.

After exiting the recrystallization chamber 17, this comes with it the resolidified silicon layer 18 and optionally an intermediate layer 9verBehene band in the covered with a quartz glass plate 24 .Doping chamber 25 · With the help of a halogen rod lamp 26 attached above the quartz glass plate 24 and one above it With a reflector 27 with ellipsoidal cross-section, the resolidified silicon layer 18 becomes superficially to a depth of 0.5 to 3 jum melted again and through the gas inlet nozzle 28, which up to approx. 0.5 to 3 cm, preferably 1 to 2 cm, reaches the re-melted point, a dopant which is opposite doped, like the already built-in dopant, for example hydrogen with 50 to 100 ppm phosphine. The amount Dopant must be calculated in such a way that not only the dopant present in the re-melted silicon layer

3 is compensated, but also additionally to 10 bis 10 Siliciumatorae a Phosphoratora is installed. Through the gas discharge nozzle 29 the reaction or residual gases are released again. The approximately 1 to 2 mm wide fused silicon layer 30 increases Phosphorus with the formation of a pn junction to the adjacent silicon layer 18 from the decomposition of the phosphine.

After passing through the gas lock 31 operated in accordance with the gas locks 5 and 12, the deposited silicon layer becomes for example with a sharp spatula 32 from the graphite plates 3 lifted off and divided into individual plates with a cutting tool 33 namely advantageously corresponding to the seams of the substrate plates.

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The graphite plates 3 returning to the new coating are superficially in a cleaning system 34, for example with a Set of steel brushes or cleaned by sandblasting and, if necessary, supplemented with new graphite plates from stock 35.

According to the method, in particular according to its preferred embodiment large areas already provided with a pn junction Self-supporting silicon plates are preferred to solar cells, which are characterized by a long service life for minority carriers * used. The achievable efficiency of such solar cells ranges up to 10%.

example

A band consisting of 10 cm wide, 10 cm long and 1 cm thick pyrographite-coated graphite plates with a rectangular cross-section in the direction of movement is composed via a pair of rollers operated by an electric motor at a speed of 5 cm / min, along two guide rails promoted by the separation system. The gas pressure in the two separation chambers is 1.2 bar, the argon pressure in the total three gas locks and 1.3 bar in the recrystallization karamer.

In the first 15 cm long separation chamber, the in this Area through a resistance heater located below the graphite plates on 1000 heated graphite substrate bodies, a 1 µm thick silicon nitride layer, by pyrolytic decomposition of the gas mixture emerging from the 2 cm above the strip made of graphite plates of monosilane in 300 parts by volume of hydrogen and ammonia in 300 parts by volume of hydrogen in a molar ratio Monosilane ammonia deposited from 1 to 3. The gas flow is 150 normal liters per hour.

In the second 100 cm long deposition chamber, which consists of a gas mixture consisting of 7-9 ° saturation gas (93 vol.% Hydrogen and 7 vol. % Trichlorosilane), to which a doping gas (hydrogen with 100 ppm diborane) is mixed in such an amount is until the separation gas

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A total of 0.15 ppm of diborane is flowed through in an amount of 1000 normal liters per hour, a silicon layer of 400 .mu.m is deposited _{on the intermediate coated Gi heated to II50: C with a resistance heater located below the graphite plates in this section of the route.}

intermediate coated graphite _{substrate body heated to II50: C}

Subsequently, the temperature of the graphite substrate body is set with a corresponding resistance heating increased to 1250 C and mediated a halogen tube lamp with a reflector with an ellipsoidal cross-section over the entire width of the graphite plate Extending approx. 1 mm narrow melt zone melted down to the intermediate layer and by counterblowing rauraterapered Argon re-solidifies on the underside of the graphite tape.

The re-solidified silicon layer is then simply supplied with energy by means of a second halogen lamp with a corresponding reflector on the surface in the presence of a doping gas containing phosphorus (hydrogen with 100 ppm phosphine) about 1 µm deep with the formation of my pn junction through the incorporation of phosphorus melted again.

The deposited silicon layer is then deposited by means of a Stainless steel spatula separated from the substrate body.

With a diamond saw this self-supporting silicon layer is following the seams of the substrate plates, in plates of 10 cm Sawn length and any adhering remnants of the intermediate layer removed by sandblasting.

After appropriate contact, the plates have an efficiency of 6 to 8 % .

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Claims (8)

-tff- claims 1. Process for the production of large, self-supporting panels of silicon, in which silicon is deposited from the gas phase on a suitable substrate body, characterized in that, that on a graphite substrate body heated to deposition temperature silicon from a gaseous compound which is additionally admixed with a dopant, is deposited to a thickness of about 200 to 650 μm and subsequently with respect to the layer height, 40 to 100% of this silicon layer melted from the free surface, by adjustment a suitable temperature gradient from the substrate body be brought to solidify again and then separated from the substrate body. 2. The method according to claim 1, characterized in that before the deposition of the Siliciuras on the substrate body, an intermediate layer is applied. 3. The method according to claim 2, characterized in that an intermediate layer of silicon nitride is applied to the substrate body will. 4. The method according to one or more of claims 1 to 3 »thereby characterized in that the gaseous compound from which silicon is deposited dopants are added which lead to lead a p-line. 5. The method according to one or more of claims 1 to 4, characterized characterized in that the gaseous compound from which silicon is deposited is a trichlorosilane-hydrogen mixture is used. 809809/0169 - 15 - OR1G! NAL INSPECTED & 263827Q 6. The method according to one or more of claims 1 to 5> characterized in that the deposited silicon layer is obtained by a radiation source attached above the silicon surface is melted. 7 · The method according to claim 6, characterized in that the Base load of the energy supply required for melting is introduced by "heating up" the substrate body. 8. The method according to one or more of claims 1 to 7, characterized in that the surface of the re-solidified silicon layer in the presence of a volatile dopant compound which doped silicon in the opposite direction to the dopants of the adjacent silicon layer to a depth of 0.5 to 1.5 / im is melted again. 9 · The method according to one or more of claims 1 to 8, characterized characterized in that the substrate body in a continuous Process is returned to its function after the deposited silicon layer has been lifted off. 809809/0169